Add link to redirect docs

[The-Compiler]

Inspired by what's done on the projects/domain_list.html template:

readthedocs.org/readthedocs/templates/projects/domain_list.html

Lines 16 to 18 in e0e6602

	{% blocktrans trimmed with docs_url='https://docs.readthedocs.io/page/custom_domains.html' %}
	Configuring a custom domain allows you to serve your documentation from a domain other than "{{ default_domain }}". <a href="{{ docs_url }}">Learn more</a>.
	{% endblocktrans %}

See #8264 (comment) for some context.

[astrojuanlu]

Thanks a lot @The-Compiler ! I'm not familiar with how translations work but this looks good to me.

[stsewd]

thanks!